A post on the BioWare Social Network by BioWare's Chris Priestly has word that EA's Origin service will indeed be a required to play Mass Effect 3, as well as word that the RPG sequel will not be sold via Steam (thanks Voodoo Extreme). Here's word:

3) Is Origin required for the retail versions of the game?
Origin is required for the PC versions of Mass Effect 3, both physical and digital.

4) Will ME3 be available on Steam?
During initial release Mass Effect 3 will be available on Origin and a number of other 3rd party digital retailers, but not on Steam at this time. Steam has adopted a set of restrictive terms of service which limit how developers interact with customers to deliver patches and other downloadable content. We are intent on providing Mass Effect to players with the best possible experience no matter where they purchase or play their game, and are happy to partner with any download service that does not restrict our ability to connect directly with our consumers.

5) Is there an opt in or opt out clause for data collection?
Users will be allowed to opt-out of Mass Effect 3 data collection from inside the game.

6) Iíve seen reports that Origin is spyware. Is this true?
Origin is not spyware, and does not use or install spyware on userís machines. In order to allow Origin to install games and their patches for everyone to use, Origin implements a permission change that results in Windows, not Origin, reviewing the filenames in the ProgramData/Origin folder. This is an ordinary Windows function, not an information-gathering process.

For instance, what if those games still there (such as dirt 2) had signed old version contracts and ended up delaying the release date until after the other games were removed? Even if no delays at all happened, if one company got the paperwork settled a year ahead of release and the other company was tardy and only got the paperwork signed weeks before release, that would explain it.

While possible, it seems fairly unlikely that the contracts for Crysis 2 and DA2 were signed after the contracts for Dirt 3 and DR2:OTR. DA2 and Crysis 2 were much higher-profile titles so it seems reasonable that EA and Valve would have finalized the contracts long before release in order to avoid potential launch issues or delays.

Another thought, perhaps there's exceptions if the DLC is exclusive to GfWL, which it appears to be the case for both Dirt 3 and DR2:OTR (I don't own either so I don't know for sure, I only spent a few mins googling about it). Valve may not want to tangle with MS over this. In contrast, the DLC for DA2 on PC was through bioware/EA's special store as I recall.

That's also possible, but why would Valve choose to tangle with EA instead of MS? EA is a much bigger player in the PC gaming market, whereas MS only releases games very rarely and nobody cares about them anyway because they're long-delayed ports of games that flopped on X360.